The gemara in nazir learns that an unspecified nezirut is 30 days long from the pasuk "kadosh yihyeh" with yihyeh having gematria 30. Can anyone think of other places where halachot are learned from gematria? I'm looking for rishonim and earlier.

5 Answers
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The Rabbis learn out that there are 39 Melachot because of a numerical value (kind of). From here (or here, if that link doesn't work), quoting Talmud Shabbat 70A:

...it says, (Shemot 35:1-2) And Moshe gathered together the entire congregation of the Children of Israel and he said to them: 'Eileh HaDevarim' (these are the things--plural) that HaShem Commanded to do them. Six days you shall do “Melacha” (creative physical activity[9]--singular), and on the seventh day, it will be to you holy, a Shabbat of Shabbats for HaShem; anyone who performs during its course (the seventh day) “Melacha”, will die.

"Devarim"; "HaDevarim"; "Eileh HaDevarim" (these are three individual superfluities of language,[10] each evoking a hermeneutic interpretation).

These (the antecedents of “Eileh HaDevarim”, which we assume Moshe was informing the Jewish people about concerning the manner by which they were to observe Shabbat) are the 39 "Melachot" that were told to Moshe at Sinai.

And the Yerushalmi (Shabbos 7:2) provides another possible way to derive this, also gematria-based: since ה and ח are often interchangeable (both being gutturals, and shaped similarly), the word אלה can be read as though it were אלח, which equals 39.
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AlexNov 24 '11 at 22:25

And by the way IIUC there is plenty of reason to suspect that this is not the original source but a parperet hachochma.
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Double AA♦May 13 '13 at 17:15

The g'mara in Mo'ed Katan 28 derives that the age of "death at the hands of heaven" is 60 from the pasuk

תָּבוֹא בְכֶלַח אֱלֵי קָבֶר כַּעֲלוֹת גָּדִישׁ בְּעִתּוֹ

in which the bold word has the numerical value of 60, yielding the interpretation

You will come to the grave at 60, like a stack that goes in its proper time

I am not sure if this satisfies the criterion "halacha" in the question, but based on the context of the g'mara (regarding specific ages of death being consequences of sins) I think an argument could be made.

In the Yerushalmi (Shekalim 1:3), Rav Yochanan ben Zakkai's position that the tribe of Levi was obligated to give the annual half-Shekel Temple tax is derived from the verse (Exodus 30:13) זה יתנו where זה is 12 in Gematria, implying all 12 tribes need to give the tax. Rambam (Shekalim 1:7) rules like this opinion.

There are lots of examples of this in Sefer Kol-Bo (c. 14th century). One such example, brought in §122, concerns the halakha that a convert will only be accepted if s/he agrees to observe the entire Torah. This is learnt out from Psalm 146:7-9, as follows:

"The Lord releases captives, the Lord opens the eyes of the blind, the
Lord straightens the bent, the Lord loves the righteous, the Lord
guards converts (שומר את גרים)": the first letters, excluding the
divine names, produce [by gematria] 613. From here it is said that one
who comes to convert on condition that he accepts all of the mitzvot
except for one, we do not accept him, for that doesn't equal the
number 613. It is for this reason that it is written, את גרים (et
gerim; with the DDOM), in order to complete the number.

(In the other four examples, there is no DDOM [definite direct object marker = את] between the verb and the noun. Were the word את to be missing from the fifth phrase as well, the gematria would equal 612. The inclusion of that extra aleph raises it to 613.)